1. Field of the Invention
The present invention relates generally to sapphyrin multimers, and derivatives and conjugates thereof. The multimers include dimers, trimers, oligomers and polymers of sapphyrin or sapphyrin derivatives, either alone, or in combination with other units such as nucleobases.
2. Description of the Related Art
Sapphyrins are large pyrrole-containing macrocyclic analogues of the porphyrins. A number of expanded porphyrin systems are now known. However, only a few fully conjugated examples have been reported that contain more than four pyrrolic subunits, namely the smaragdyrins, sapphyrins, pentaphyrins, hexaphyrins, and superphthalocyanines.sup.1 (Sessler & Burrel, 1991).
Sapphyrin, first discovered serendipitously by Woodward.sup.2 is one of the more intriguing products to emerge from initial studies directed towards the synthesis of Vitamin B.sub.12..sup.2,3 It is a 22 pi-electron pentapyrrolic macrocycle which exhibits an intense Soret-like band at about 450 nm (CHCl.sub.3) along with weaker Q-type transitions in the 620 to 690 nm region. These optical properties, along with the presence of a large central cavity which serves for metal binding, renders sapphyrin useful for certain biomedical applications, including photodynamic therapy (PDT) and magnetic resonance imaging enhancement (MRI).
In addition to the above, certain expanded porphyrins, including especially those of the sapphyrin series, have been found to act as halide anion chelating agents in both solution and the solid state.sup.4. This finding, along with an appreciation that the diprotonated form of 3,8,12,13,17,22-hexaethyl-2,7,18,23-tetramethylsapphyrin acts as an efficient carrier for the through-dichloromethane-membrane transport of nucleotide monophosphates, such as e.g. guanosine-5' monophosphate, and related entities at acidic pH.sup.5 led the inventors to consider that the basic sapphyrin structure and related compounds such as the rubyrins, if suitably modified, could be used to bind, recognize, and transport phosphorylated entities at or near neutral pH.
Unfortunately, all sapphyrins known at the time of this invention were known both to be essentially insoluble in water and also known to be ineffective as through membrane carriers for phosphate monoesters including those specifically that define the class of compounds known as nucleotides and nucleotide analogues.sup.5. In addition, the sapphyrins known prior to the present invention were all of such simple character in terms of peripheral substituents, such that only hydrogen or alkyl were known.sup.2,3. These deficiencies limited the potential utility of sapphyrins for any applications associated with their use at or near neutral pH and, more generally, any conditions involving partial or complete association with an aqueous environment.
At present, no general set of nucleotide transport agents exists.sup.10. In early work Tabushi was able to effect adenosine nucleotide transport using a lipophilic, diazabicyclooctane-derived, quaternary amine system.sup.10a. However, this same system failed to mediate the transport of guanosine 5'-monophosphate (GMP) or other guanosine-derived nucleotides. Since then, considerable effort has been devoted to the generalized problem of nucleic acid base ("nucleobase") recognition, and various binding systems have been reported.
Currently known nucleotide binding systems include various acyclic, macrocyclic, and macrobicyclic polyaza systems.sup.10a-10n ; nucleotide-binding bis-intercalands.sup.10k ; guanidinium-based receptors.sup.10f,10n ; and various rationally designed H-bonding receptors.sup.10o-10u. These latter H-bonding receptors have been shown to be effective for the chelation of neutral nucleobase and/or nucleoside derived substrates but, without exception, have all proved unsatisfactory for the important task of charged nucleotide recognition. Thus, despite intensive efforts in this field, there is currently no synthetic system capable of effecting the recognition, or through-membrane transport, of phosphate-bearing species such as anti-viral compounds. Furthermore, there are presently no rationally designed receptors which are "tunable" for the selective complexation of a given nucleobase-derived system.
Not surprisingly, the transport of larger polyphosphorylated compounds across cellular membranes also poses significant problems. The difficulties in transporting oligonucleotides across the plasma membrane and into mammalian cells is one of the factors currently limiting the successful application of antisense technology to human therapy. Further limitations may also result from the dynamics of oligonucleotide recognition, binding and functional inhibition which occurs intracellularly, subsequent to any import that does occur.
There is clearly, therefore, a major need for novel drug delivery systems to be developed. Compounds which would allow negatively-charged (anionic) structures, particularly phosphate-bearing compounds, including nucleotides and nucleotide derivatives such as anti-viral compounds and anti-sense oligonucleotides, to be transported across naturally lipophilic cellular membranes would represent an important scientific and medical advance.
In addition to the delivery of compounds to cells, there still remains considerable scope for the design of improved chemotherapeutic compounds which act upon DNA once inside a target cell. Since currently available chemotherapeutic agents have complex structures, or complicated modes of interaction with their targets that preclude systematic improvement, the development of a novel class of DNA binding compounds would open up new avenues for the design of improved therapeutics. In this regard, a class of compounds that can be modified in a number of different ways while maintaining their overall oligomeric, or polymeric structure would be particularly advantageous. The same is true for compounds that can be activated by light, or other means, to produce singlet oxygen or hydroxyl radicals, once bound to DNA. These considerations provided the present inventors with further impetus for the design and synthesis of improved sapphyrins such as those embodied by the present invention.